1. Field of the Invention
The present invention generally relates to an image reading technique and an image forming and reproducing apparatus using an image reader, and more particularly, to an image reader using an image sensor that groups analog pixel signals representing sensed light quantities into at least two parts (the first part and the last part) in the fast scan direction and outputs the grouped signals in different channels.
2. Description of Related Art
A linear image sensor is generally used in an image reader of a digital copier or a facsimile machine, or in an image scanner. A conventional CCD linear image sensor is configured to output odd-number pixel data and even-number pixel data alternately, dividing output signals into an odd channel and an even channel. The image reading speed is increased by providing a combination of such an ODD/EVEN 2-channel image sensor and an analog signal processor to process the two lines of analog pixel signals output from the image sensor in parallel.
However, needs for image readers capable of faster reading operations are now arising, and therefore, it is required to further improve the reading speed over the conventional ODD/EVEN 2-channel image sensor.
In response to this demand, a 4-channel output image reader that quarters the pixel frequency to realize double the reading speed of the conventional ODD/EVEN 2-channel output image sensor is proposed. With the 4-channel output image reader, each line of light-receiving elements (or light-to-electric converting elements) is divided at the center into the first part and the last part in the fast scan direction, in addition to the grouping of odd-number pixels and even-number pixels. Thus, the entire pixels are divided into four groups, and the sensed signals are output through four channels. This type of image sensor is referred to as an FL-type image sensor, and disclosed in, for example, Japanese Patent Laid-Open Publication Nos. 11-215298A and 2002-158837A.
In the image reader using a 4-channel output image sensor, if a level difference remains between the left-region scanning signal and the right-region scanning signal due to a slight difference in linearity characteristics of the output signals on the four channels, the image quality may differ between the left part and the right part divided by the FL boundary position. Signal level difference occurring between the odd-number pixels and the even-number pixels of a conventional ODD/EVEN 2-channel is not so serious because only a very minute repeated pattern is added to the reproduced image. Such a slight variation in the fine and repeated pattern does not influence the image quality very much. In contrast, if a slight amount of signal level difference is generated between the left half and the right half of the image, such a small level difference will result in conspicuous image degradation.
To overcome this problem, Japanese Patent Laid-Open Publication No. 2002-218186 proposes a technique for bringing the signal levels of the F-channels and the L-channels of a 4-channel output image sensor in agreement with each other by performing analog-to-digital conversion and shading correction for each of the first-part even-pixel (FE) group, the first-part odd-pixel (FO) group, the last-part even-pixel (LE) group, and the last-part odd-pixel (LO) group, independently, and by further performing gamma correction on the corrected data using a lookup table.
FIG. 1 is a block diagram of a conventional image reader that performs the above-described image correction. The image reader includes a CCD 101, which functions as the 4-channel output image sensor configured to output analog pixel signals through the FE, FO, LE, and LO channels in parallel. The FE channel and the FO channel are connected to the associated analog processing LSI circuit 102. The LE channel and the LO channel are connected to the associated analog processing LSI circuit 103. The data of each channel are amplified to a prescribed level under automatic gain control (AGC) of the amplifier at the associated analog processing circuit 102 or 103. Then, the data items on the even channel and the odd channel are combined and rearranged in a time-series signal sequence using a multiplexer, and subjected to other necessary analog signal processing. Then, the first-part (F-channel) data and the last-part (L-channel) data are supplied to the associated analog-to-digital converters 104 and 105, respectively, and converted into digital form. The digitized F-channel data and L-channel data are subjected to shading correction at the shading correction circuits 106 and 107, respectively, which circuits 106 and 107 are structured by memories and arithmetic components. Then, the F-channel and L-channel digital data items are supplied to pixel rearrangement means (not shown), and combined and rearranged in a time-series signal sequence. The combined signal is output from the image reader after necessary image processing.
The gains of the amplifiers of the analog processing circuits 102 and 103 are adjusted automatically, by detecting the levels of the digital data items output from the A/D converters 104 and 105 and calculating the corresponding gains at the calculation unit 108. The calculated gains are fed back to the analog processing LSI circuits 102 and 103, respectively, so as to perform automatic gain control.
A gamma correction table 109 is provided to one of the F-channel and L-channel digital data paths so as to bring the linearity characteristic of the digital data item of one channel into agreement with that of the other channel. In FIG. 1, gamma correction is performed on the L-channel. Thus, the first-part odd and even analog signals and the last-part odd and even analog signals are amplified by the amplifiers of the analog processing circuits 102 and 103, respectively, to a prescribed level, and converted to a digital form at the A/D converters 104 and 105, respectively. After the shading correction at the shading correction circuits 106 and 107, gamma correction is performed at the gamma correction table 109 so as to make the linearity characteristics of the F-channel and L-channel digital data items consistent with each other.
Difference between the even-pixel group and the odd-pixel group, and difference between the first-part group and the last-part group may occur due to variation in the signal processing circuits. However, the major factor of such differences resides in variation in the linearity characteristic of the output signal itself from the CCD 101. The linearity characteristic represents the analog output (scanner output) with respect to the incident light quantity of the CCD 101. Since the analog output is in proportion to the light quantity, it should become linear, logically. However, in the actual operation, the analog output may not be linear, as illustrated in FIG. 2, and there may be difference in linearity characteristic between the F-channel data and the L-channel data.
In view of the linearity difference in the actual operation, the conventional difference correction method disclosed in, for example, 2002-218186 cannot follow the change in the CCD output, such as change in the incident light quantity over time. This is because the amplifiers used in the analog processing circuits 102 and 103 are AGC amplifiers, and because such amplifiers automatically amplify the input signals to a prescribed level regardless of the CCD output values. Analog-to-digital conversion and shading correction are performed on the uniformly amplified analog data, and then gamma correction is performed using the gamma correction table 109. The gamma correction table 109 is generally created under the initial condition of 100% light quantity. But such a factory default gamma correction table 109 may be used as it is under the lowered light quantity of, for example, 50%.
FIG. 3 is a block diagram of a conventional image reader with another structure. In FIG. 3, analog pixel signals are output from the CCD 201 through the FE, FO, LE, and LO channels in parallel. The FE channel and the FO channel are connected to the associated analog processing LSI circuit 202. The LE channel and the LO channel are connected to the associated analog processing LSI circuit 203. The data of each channel are amplified to a prescribed level under automatic gain control (AGC) of the amplifier at the associated analog processing circuit 202 or 203. Then, the data items on the even channel and the odd channel are combined and rearranged in a time-series signal sequence using a multiplexer, and subjected to other necessary analog signal processing. Then, gamma correction is performed on the digital data of one of the first-part and last-part channels (for example, the L-channel in the example shown in FIG. 3) so as to make the L-channel digital data consistent with the F-channel digital data, using a lookup table 206 formed by a ROM in which the gamma correction data are written. Then, the F-channel and L-channel digital data items are combined and rearranged in a time-series signal sequence at the pixel rearrangement LSI circuit 207, subjected to shading correction at the shading correction circuit 208, and output from the image reader.
The difference between the F-channel and the L-channel of the FL type CCD 201 is corrected by performing gamma correction on the digitized image data, using the lookup table 206. However, the gamma correction values may become inappropriate due to change over time in temperature or other factors, and the difference between the F-channel and the L-channel may becomes conspicuous even if the lookup table 206 is appropriately adjusted from the factory default initial condition. In this case, it is desired that the gamma correction values be readjusted by service persons or users; however, no countermeasures for this problem are addressed.
In addition, none of the above-listed publications describe the detailed process of acquiring output values of each area when reading multiple gray charts or interpolation among gray charts required during the creation of the lookup table. If there is a slight amount of level change occurring in output signals at the boundary between the first part and the last part, the level difference is perceived in the reproduced image. Since inappropriate calculation of the area output values or inappropriate interpolation will result in conspicuous image degradation, the conventional technique is insufficient to adjust the output levels of a 4-channel output image sensor.